Like general diodes, Light Emitting Diodes (LEDs) are turned on when a forward voltage having the level greater than a threshold voltage is applied thereto. When turned on, the LEDs conduct current and emit light.
Serial type LEDs, directly driven by voltage from an Alternate Current (AC) power source (or a source voltage) are connected to a full-wave rectifier, in which case, one or more LEDs are connected in series, or are in a combination of serial and parallel connections.
The LEDs are turned on and current flows through them when a voltage greater than the threshold voltage is applied thereto, but they are turned off and no current flows through them when a voltage applied is less than the threshold voltage.
For a cycle of the source voltage, the fact that time for which the LEDs are turned on is short reduces the luminous intensity of the LEDs and causes total harmonic distortion.
As the number of LEDs connected in series increases, the level of the voltage required to turn on the LEDs also increases and the time for which the LEDs are turned on gets shorter. This degrades the luminous intensity and worsens the total harmonic distortion, leading to an increase in manufacturing cost.
On the other hand, as the number of the LEDs connected in series decreases, the level of the voltage required to turn on the LEDs is reduced but overcurrent may flow through the LEDs, shortening the lifespan of the LEDs. In this case, voltage fluctuation of the AC power source may also cause the overcurrent.
There is a need to develop an apparatus for driving LEDs that is not affected by voltage fluctuation of a power source, increases luminous intensity of the LEDs, reduces the total harmonic distortion, prevents overcurrent, and reduces manufacturing costs.
FIG. 1 shows an LED driver circuit, in which a power source AC, a diode rectifier unit Dr, a current regulating resistor Rr, and an LED unit De having multiple LEDs are connected in series.
FIG. 2 shows waveforms of voltage (or source voltage) VAC and current IAC of the AC, a rectified voltage Vcc by the diode rectifier unit Dr, and a rectified current Icc flowing through the LED unit De.
Referring to FIG. 1, the source voltage VAC applied across the diode rectifier unit Dr is full-wave rectified and the rectified voltage Vcc is applied to the LED unit De via the resistance Rr.
When the rectified voltage Vcc is less than a total forward threshold voltage Vth1 for the LED unit De having multiple LEDs (i.e., the sum of forward threshold voltages for the respective LEDs), the LED unit De is turned off for certain time periods t1 and t3, as shown in FIG. 2, and current Icc does not flows though the LED unit De.
On the other hand, when the rectified voltage Vcc is greater than the forward threshold voltage Vth1 (for time t2), the LED unit De is turned on and current Icc starts to flow through the LED unit De, the level of the current Icc being equal to a value obtained by dividing the difference between the rectified voltage Vcc and the forward threshold voltage Vth1 by the resistance of the resistor Rr. As the rectified voltage Vcc increases, a problem arises that the current Icc flowing through the LED unit De may increase to more than a maximum permissible current.
As the level of a voltage required to turn on the LED unit De, i.e., the level of the forward threshold voltage Vth1 increases in proportion to the number of the LEDs connected in series, the turned-on section of the LED unit De becomes short.
This accentuates the total harmonic distortion and degrades the luminous intensity of the LED unit De.
When the forward threshold voltage Vth1 for the LED unit De is reduced or the source voltage VAC increases, a current greater than a permissible current may flow through the LED unit De, thus leading to reduction of the lifespan of the LED unit De and reliability of its operation.
The total harmonic distortion is a cause of various electric noises and is thus subject to global regulation. When the luminous intensity of the LED unit De decreases, more LEDs are required to make up for the decrease of the luminous intensity, which in turn increases the manufacturing cost.
FIG. 3 shows an LED driver circuit that improves the total harmonic distortion.
Referring to FIG. 3, a power source AC, a resistor R for current regulation, a first LED unit Da, and a second LED unit Db are connected in series. The first LED unit Da comprises two LEDs Da1 and Da2 connected in an inverse parallel form, and the second LED unit Db comprises two LEDs Db1 and Db2 also connected in an inverse parallel form.
The LED driver circuit of FIG. 3 introduces a capacitor C1 connected between the junction na of the resistor R and the first LED unit Da and the junction nb of the first and second LED units Da and Db, to improve the total harmonic distortion, and the source voltage VAC is applied across the first and second LED units Da and Db via the resistor R without any rectifier.
FIG. 4 shows waveforms of the source voltage VAC and current IAC applied from the source power AC, a voltage VR after the resistance R, a current IDa flowing through the first LED unit Da, and a current IDb flowing through the second LED unit Db.
When the level of the voltage VR is less than a forward threshold voltage Vth2, no current flows through the first LED unit Da, and when the voltage VR is equal to or greater than the forward threshold voltage Vth2, current IDa2 flows through the forward LED Da2 of the first LED unit Da for a positive (+) half cycle of the source voltage VAC and current IDa1 flows through the backward LED Da1 of the first LED unit Da for a negative (−) half cycle of the source voltage VAC, the sum of the currents IDa1 and IDa2 being the current IDa of the first LED unit Da.
With the voltage VR less than the forward threshold voltage Vth2 applied, when the source voltage VAC increases in the positive direction, a charging current flows through the second LED unit Db via the capacitor C1, and even when the source voltage VAC increases in the negative direction, a discharging current flows through the second LED unit Db via the capacitor C1.
When the voltage VR is equal to or greater than the forward threshold voltage Vth2, current IDb2 passed through the forward LED Da2 of the first LED unit Da flows through the forward LED Db2 of the second LED unit Db for a positive (+) half cycle of the source voltage VAC and current IDb1 passed through the backward LED Da1 of the first LED unit Da flows through the backward LED Db1 of the second LED unit Db for a negative (−) half cycle of the source voltage VAC. The sum of the currents IDb1 and Idb2 is the current IDb of the second LED unit Db.
However, when the source voltage VAC is falling, no current flows through the capacitor C1 and when the voltage VR is less than the forward threshold voltage Vth2, no current flows through the first and second LED units Da and Db.
When the source voltage VAC is on the rise, charging current or discharging current is produced in the capacitor C1 by means of the second LED unit Db, which may reduce the total harmonic distortion to a certain extent. However, the capacitor C1 has a short lifespan and needs to endure high voltage, and it is difficult to make a product including the capacitor C1 compact due to the cost problem and the size of the capacitor C1.
The charging current or discharging current flows only through the second LED unit Db thus half of all the LEDs used to reduce the total harmonic distortion, which makes the current flowing through the second LED unit Db greater than the current flowing through the first LED unit Da. In this case, when a maximum current is supplied to the first LED unit Da, the second LED unit Db conducts overcurrent. This makes it hard to supply a current sufficient to drive the first and second LED units Da and Db of the LED driver circuit.
It is not possible to pass maximum permissible current for LED through the first and second LED units Da and Db, which reduces the luminous intensity.
In addition, when the source voltage VAC increases due to fluctuations, the level of the current flowing through the first and second LED units Da and Db may rise to more than the maximum permissible current. The current flowing through the LEDs may not reach the maximum permissible current in consideration of the fluctuation of the source voltage VAC, thus reducing the luminous intensity.